1. Field of the Invention
This invention relates to an electrophotographic photosensitive member which can be utilized widely for electrophotographic field of application. More particularly, it pertains to a highly durable electrophotograhic photosensitive member excellent in humidity resistance and mechanical strength.
2. Related Background Art
An electrophotographic photosensitive member is required to have necessary sensitivity, electrical characteristics, optical characteristics corresponding to the electrophotographic process to be applied, and further in a photosensitive member which is to be used repeatedly, since the surface layer of the photosensitive member, namely the layer the remotest from the substrate, is applied directly with electrical, mechanical external force such as by corona charging, toner development, transfer to paper, cleaning treatment, etc., durability to those is required.
Specifically, durability to generation of abrasion or damage on the surface by sliding, or deterioration of the surface by ozone generated during corona charging, etc. has been demanded.
On the other hand, there is also a problem of toner attachment onto the surface layer by repeated development of toner and cleaning, and to cope with this problem, improvement of the cleaning characteristic of the surface layer has been demanded.
Various methods have been investigated in order to satisfy the characteristics required for the surface layer as described above, and among them the means of dispersing fluorine type resin powder in the surface layer is effective, as disclosed in Japanese Laid-open Patent Applications Nos 25749/1981, 126836/1981, 74748/1982 or USP 4663259. By dispersion of fluorine type resin powder, frictional coefficient of the surface layer is lowered to act on improvement of cleaning characteristic, improvement of durability to abrasion resistance, etc.
Such means is also effective for prevention of surface deterioration under humid conditions, since water repellency and mold releasability can be also improved.
However, in the fluorine type resin powder dispersion, there is a problem involved in its dispersibility and agglomerating characteristic. That is, since a uniform and smooth film can be formed with difficulty, the surface layer obtained cannot avoid inevitably having image defects such as image irregularity, pinhole, etc.
Although some of binding materials resins or dispersing aids can disperse fluorine type resin powder uniformly to form a smooth film, in most cases, they have hydroxyl groups, carboxyl groups, ether bonds, etc., and therefore deterioration of electrophotographic characteristics will be caused by carrier trap under high temperature and high humidity conditions, whereby no practically applicable one can be found under the present situation.
In contrast, the present Applicant has previously found in Japanese Patent Applications Nos. 58253/1986 and 54096/1987 that an electrophotographic photosensitive member having the surface layer comprising fluorine type resin powder dispersed in the presence of a fluorine type graft polymer can respond to the requirements as mentioned above.
However, a fluorine type graft polymer requires a large number of reaction steps in synthesis. The present Applicant has reached the present invention as the result of investigation of a dispersing aid which can be synthesized according to simpler method and can exhibit an effect equal to or better than that as described above.